1. Field of the Invention
The present invention is related to the field of geophysical exploration. More specifically, the present invention is related to the use of borehole seismic sensor systems.
2. Description of the Related Art
A borehole seismic sensor system is an instrument adapted for traversing a wellbore penetrating the earth, which is used for converting seismic energy into a signal representing the time-varying amplitude of the seismic energy, after the seismic energy has passed through the earth to the wellbore.
Borehole seismic sensor systems are commonly used for conducting checkshot and vertical seismic profile, or VSP, surveys. Checkshot and VSP surveys are typically used to correlate a geophysical survey conducted at the earth's surface to the depth of various formations in the earth. For example, "Vertical Seismic Profiling", by Bob A. Hardage, Geophysical Press, London, 1985, describes the methodology and applications of checkshot and VSP surveys in detail.
A borehole seismic sensor system must be able to function while withstanding the environment in the wellbore, which can frequently include temperatures exceeding 300 degrees Fahrenheit, and pressures exceeding 15,000 PSI. A borehole seismic sensor system adapted to withstand the environment of the wellbore can be relatively heavy. Efficient transfer of seismic energy requires good acoustic coupling of the seismic sensor system to the wellbore wall. Good acoustic coupling is facilitated by designing the seismic sensor system so that the weight of the system be kept as small as possible while still being able to withstand the environment in the wellbore.
A clamping mechanism is typically part of the borehole seismic sensor system. The clamping mechanism forces a housing, which forms part of the system, into contact with the wall of the wellbore so that friction between the housing and wellbore wall causes acoustic coupling of the housing to the wall of the wellbore. The housing can contain sensors which detect seismic energy by generating a signal in response to acoustically induced motion of the housing. Seismic energy reaching the wellbore must be transferred to the housing entirely by the friction between the wall of the wellbore and the housing in order to be detected by the sensors, so the clamping mechanism must generate enough force both to support the entire weight of the system in the wellbore and to enable coherent transfer of seismic energy from the wall of the wellbore to the housing.
Inadequate clamping force may enable relative movement between the housing and the wellbore wall, which can result in distortion of the seismic signal detected by the sensors in the housing. The requirement for an adequate amount of clamping force is described, for example, in "Vertical Seismic Profiling" (page 36). Clamping mechanisms which provide adequate force for holding the housing in contact with the wall of the borehole are known in the art. For example, "The AWS-1300G Downhole Geophone" Atlas Wireline , Services, Houston, Tex., 1993, describes a clamping mechanism which generally provides adequate clamping force.
Even if the clamping force generated by the clamping mechanism is generally adequate to provide good acoustic coupling of the system to the wellbore wall, the actual quality of acoustic coupling may vary within any particular wellbore, because the acoustic coupling quality is dependent on the friction between the housing and the wellbore wall, not on the clamping force alone. The friction which is actually generated by the clamping mechanism forcing the housing against the wellbore wall can be heavily dependent on the condition of the wellbore wall. A rough, or rugose wellbore wall can be caused by various conditions encountered while drilling the wellbore, which include soft earth formations. A rugose wellbore wall can cause the contact surface area between the housing and the wellbore wall to be significantly less than with a smooth wellbore wall, thereby resulting in inadequate friction for good acoustic coupling.
It is desirable to be able to determine the quality of acoustic coupling of the system to the wall of the wellbore at a particular position within the wellbore, so that if the particular position is determined to have poor quality acoustic coupling, then the system can be moved to another position which may have better quality coupling.
A method for determining the quality of the acoustic coupling between the wellbore wall and the housing is known in the art. The method known in the art uses an additional geophone receiver, located within the housing, to act as a "shaker", whereby an electrical signal generated either at the earth's surface or in the system itself, is converted by the additional geophone receiver into an oscillating mechanical force which is applied to the system. The quality of the acoustic coupling between the housing and the wellbore wall is determined by analyzing the response of the system to the oscillating force applied by the shaker.
The method known in the art is difficult because the additional geophone receiver must be added to the system. The additional weight of the extra geophone receiver, associated signal processing circuitry and the larger housing needed to enclose the extra geophone receiver and circuitry, all add to the weight of the system which must be clamped to the wellbore wall, thereby making good acoustic coupling more difficult.
It is an object of the present invention to provide a shaker means, or means of exciting the borehole seismic sensor system without an additional geophone receiver and associated circuitry, so that the mass of the borehole seismic sensor system is not substantially increased over the mass of a system without an integral shaker means.